As is known in the art, a coplanar waveguides (CPW) structure includes: a center conductor disposed over a surface of a substrate; and a pair of ground plane conductors disposed over the surface of the substrate, the center conductor being disposed between the pair of ground plane conductors. Microwave energy fed to an input of the CPW propagates to an output in a differential transmission mode relative to the pair of ground plane conductor with the electromagnetic field being near the surface substrate. CPW has been and continue to be used in wide variety of integrated circuit and circuit board applications. However, being a three conductor system, CPW structures are vulnerable to propagation of unwanted common mode(s). For example, in many applications the integrated circuit having active elements interconnected on a top, or upper, surface of a common substrate and a conductor is disposed on the bottom surface of the substrate for mounting to a heat sink or to a system ground conductor, for example. In this example, a parallel plate region is formed between the conductors on the upper surface, particularly, when larger ground plane conductors are used for the CPW transmission line, and the conductor on the bottom surface.
More particularly, a microwave parallel plate region includes a pair of conductors disposed over opposite surfaces of a substrate. When such parallel plate region is used as a portion of a microwave transmission line, unwanted, parasitic, parallel plate modes may be generated (moding), supported between the pair of conductors, and then transmitted through the parallel plate region. In one application, a substrate may be used to realize a Monolithic Microwave Integrated Circuit (MMIC) chip having an amplifier with a conductor on the bottom of the substrate, for providing a system ground or for soldering to a printed circuit board or heat sink, for example, and conductors on the top of the substrate. In such a chip, transmission lines are used to interconnect elements of the amplifier. As a result of the top and bottom conductors, parallel plate moding may be generated. If the generated moding has frequencies within the bandwidth of the amplifier with magnitudes equal to, or greater than, the forward gain of the amplifier, a portion of the output energy produced by the amplifier may be coupled back to the input of the amplifier providing positive feedback thereby generating unwanted oscillations.
Common mode generation may also result from interference from other sources, such as, for example, coupling of external signals generated by other sources, unbalanced excitation or unbalanced ground paths.
Thus, while CPW transmission uses a differential mode transmission, these other sources can generate common modes that can propagate through the CPW transmission lines as unwanted signals and become a source of parasitic unwanted common mode signals that propagate through the one or more of the center conductors and pair of ground plane conductors and adversely affect the performance and operation of the MMIC.